1. Field
The invention disclosed and claimed herein generally pertains to a spirally wrapped twinax cable for transmitting information, such as digital data or other information produced by a data processing system. More particularly, the invention pertains to a cable of the above type that may have a length of 6-10 meters or greater.
2. Description of the Related Art
As is known by those of skill in the art, a twinax transmission cable has two conductors that are placed side by side along the length of the cable, in closely spaced relationship. The two conductors are surrounded by insulation and an outer shielding layer, and the cable may also include a DC drain conductor. Twinax cables are currently used to carry digital information signals between components of computers and data processing systems. When used for such applications, one of the conductors functions as the source conductor, and the other functions as the return conductor. Of course, during AC transmissions the roles of the two conductors switch continually.
In fabricating twinax cables, it is common practice to apply the outer shielding layer by means of spiral wrapping, in an effort to keep down fabrication costs. In spiral wrapping, a tape or thin strip of shielded foil or the like is wrapped spirally around the conductors and insulation, along the length of the cable. Each time an individual wrap is made around the conductors and insulation, a portion of the wrap is placed on top of a portion of the adjacent previous wrap. This aspect of spiral wrapping is essential, to ensure that there are no gaps in the shielding, between the edges of two adjacent wraps.
At present, twinax cables are known to have certain advantages over other transmission media, when used to transmit digital data and other information signals in a data processing environment. For example, signal attenuation due to dielectric loss is significantly less for a twinax cable than for a printed circuit board, particularly for operation at high frequencies. Notwithstanding these benefits, however, currently used designs, and in particular the use of spiral wrapping as described above, has been found to place undesirable limitations on the use or employment of conventional twinax cables.
To understand these limitations, it must be appreciated that as a result of the spiral wrapping technique, a portion or segment of a shielding wrap overlays or overlaps a segment of the previous adjacent wrap. Moreover, these overlap conditions or instances occur at fairly regular, or periodic, intervals along the length of the cable. Accordingly, a discontinuity occurs at each of the overlap conditions. Since the shielding foil also acts as a current return path when current flows in both the conductors, the return current flow through the shielding wrap does not exactly match the outgoing current through conductors. The discontinuity at an overlap is caused by an LC resonance effect, wherein the capacitance for the effect is provided by the two adjacent wrap segments of the overlap condition. Attenuation resulting from this LC effect, particularly at or near the resonance frequency, can significantly diminish signal transmission integrity and quality along the cable. Moreover, the LC resonance effect increases with cable length.
The terms “spirally wrapped” and “spirally wrapped in an overlapping maneuver”, as used herein, refer to a spiral wrapping technique as described above.
Since the above deficiency of twinax cables arises from the practice of spiral wrapping, an alternate technique could be used to apply shielding to twinax cables, that avoided the creation of shielding overlaps. However, such alternate shielding techniques as are currently available typically add substantial cost to twinax cable construction, and in particular to cables having lengths of three or more meters. Accordingly, this solution is not practical for many user applications.